1. Field of the Invention
The invention relates to a multi-ply filter laminate/composite for manufacturing vacuum cleaner filter bags, comprising at least one ply of filter paper and at least one ply of fine fibered web.
2. Discussion of the Prior Art
To improve the filtering properties, a large proportion of the filter bags manufactured presently are double-walled with a filter paper outer ply and a highly porous web inner ply. The latter is in most cases a cellulose version, which in certain cases can also be admixed with synthetic fibers. A particular embodiment of the double-walled vacuum cleaner filter bag contains a filter paper outer ply and a melt-blown fine fibered web as the inner ply in order that extremely high values of filtering efficiency may be obtained (DE-38 12 849 C2, EP-0 338 479 BP). To produce the bag, the bagmaking machine is supplied with the paper and the melt-blown web from separate rolls. The paper (1) and web (2) are pulled into the bagmaking machine with their edges offset in order--as depicted in FIGS. 1a and 1b that paper-to-paper bonding (3) and web-to-web bonding (4) may be ensured in the longitudinal seam region.
If this edge offset were not used, a melt,blown fiber web layer would come to lie between the plies of paper to be bonded together and would--since the web is made of plastic-act as a separating layer and make it very difficult, if not impossible, to achieve adequate longitudinal seam bonding. Melt-blown webs can in principle be made of thermoplastics, preferably polyolefins, polyamides, polyesters, polycarbonate and copolymers thereof. Melt-blown webs, provided they are of the appropriate fiber fineness between 2 and 10 .mu.m in diameter, are excellent filter materials, but they have only limited mechanical properties in strength. Owing to the low strength values, the melt-blown web, which in processing is pulled into the bagmaking machine from a separate roll, has a lower limit as regards the weight per unit area. Thus, the weights employed in known embodiments are usually 15-35 g/m.sup.2. If the weight is below 15 g/m.sup.2 it is admittedly possible to exert an influence over the strength formation (bursting pressure, breaking resistance) in the construction of the melt-blown web, but the porosity of the material is very adversely affected, so that use as a filter material is ruled out. For this reason the melt-blown webs used today for this purpose represent a compromise between contradictory, product features, namely filtering and mechanical properties, so that optimum capabilities in filtering properties are very difficult to achieve.
However, if prior to processing on the known bagmaking machines this melt-blown material is combined with the contemplated filter paper outer ply to form a laminate/composite, the mechanical properties of the melt-blown web no longer have to meet any strength requirements, since they are met by the filter paper. The construction of the melt-blown web can for this reason be fully focused on the optimization of the filtering properties, so that even highly porous, open-structured filter webs can be employed. The weight per unit area is limited at the lower end merely by the stipulated filter efficiency and the storage capacity of the material; the strength of the material in terms of the bursting pressure or the breaking resistance does not play a part in this form during processing. The weight per unit area can therefore be found to be far below 10 g/m.sup.2.
However, with the applied melt-blown layer covering all of the paper, the disadvantage of these embodiments in bag manufacture is that, in the operation of bag longitudinal seam bonding (3'), a melt-blown layer (2') now comes to lie between the paper plies (1'), which after all have to be firmly bonded together (see FIGS. 2a and 2b). This melt-blown layer (2') between the paper plies (1') prevents sufficiently strong longitudinal seam bonding (3') using conventional types of adhesive. Even if other adhesive bonding techniques that are not customary in this industry are used, satisfactory adhesion between the paper plies (1') is not always achievable.